WO 99/60341 describes an optical sensor, fabricated using conventional micromachining techniques, for measuring changes in temperature and pressure inside a combustion engine. The sensor comprises a slab of silicon having a recess defined in one surface by etching. A micro-capillary, having a silica fibre fixed inside, is adhered to the silicon slab so as to close the recess. The face of the silica fibre and the inner surface of the recess directly opposite the fibre serve to define a Fabry-Perot cavity. Light incident along the silica fibre is reflected within the Fabry-Perot cavity and guided back along the silica fibre. The reflected light creates interference fringes whose characteristics are determined by the length of the Fabry-Perot cavity. Changes in the external pressure cause the wall of the silicon slab directly opposite the fibre to deflect, causing a change in the length of the Fabry-Perot cavity. This in turn creates a change in the characteristics of the interference fringes thus registering a change in pressure. The sensor may also be used to sense changes in temperature by employing a suitably thick slab of silicon. Changes in temperature cause the slab to expand or contract, which in turn results in a similar expansion or contraction of the Fabry-Perot cavity.
Whilst the silicon sensor may be used for many applications, the sensor is unsuitable for environments that are at elevated temperatures. In particular, the maximum temperature at which the silicon sensor can operate is around 450° C. Above this temperature, the elastic properties of silicon become unstable making any measurements unreliable.
WO 2005/098385 describes a sapphire optical sensor sensitive to both pressure and temperature. A waveguide formed from an optical fibre, hollow ceramic rod or metal tube is used to interrogate the optical sensor. The waveguide is bonded directly to the optical sensor using one of a number of bonding techniques. In one embodiment a sapphire optical fibre is fusion bonded to the optical sensor at temperatures between 600° C. and 1500° C.
In an alternative embodiment described in WO 2005/098385, the waveguide is spaced from the optical sensor by a short distance of around 3-100 μm. However, this sensor is not suitable for use at elevated temperatures as the fusion bond between the waveguide and the optical sensor will weaken and may fail between 600° C. and 1500° C.
US 2007/0013914 describes an optical fibre sensor formed by bonding a sapphire membrane to the end of a capillary tube and bonding an optical fibre within the capillary tube so that the end of the optical fibre and the near (to the optical fibre) surface of the sapphire membrane define a first optical cavity. The optical fibre may be bonded with epoxy or laser welded to the capillary tube.
A second optical cavity is defined by the near and far surfaces of the sapphire membrane and is used to obtain a compensating temperature measurement. However, due to thermal mismatch between the optical fibre and capillary tube this sensor is not suitable for use at high temperatures.
These prior art devices are not suitable for use at elevated temperatures or to be cycled repeatedly from low to high temperatures without structural damage due to thermal mismatch. Therefore, there is required an optical sensor that overcomes these problems.